The present invention relates to a method for preparing a hologram recording medium and particularly relates to a method for preparing a hologram recording medium, with which a gradated motif expression is enabled by computation using a computer.
Holograms are widely used in applications for preventing counterfeiting of cash vouchers and credit cards. Normally, a region onto which a hologram is to be recorded is set up in a portion of a medium to be subject to counterfeiting prevention, and a hologram of a three-dimensional image, etc., is recorded inside this region.
Although conventionally with many commercially utilized holograms, an original image is recorded onto a medium in the form of interference fringes by an optical method, methods for preparing a hologram by forming interference fringes on a recording surface by computation using a computer have come to be known recently. A hologram prepared by such a method is generally referred to as a “computer generated hologram (CGH)” or simply as a “computer hologram.” A computer hologram is obtained by simulating a so-called optical interference fringe generating process on a computer, and an entire process of generating interference fringe patterns is carried out as computation on the computer. Upon obtaining image data of interference fringe patterns by such a computation, physical interference fringes are formed on an actual medium based on the image data. As a specific example, a method, with which image data of interference fringe patterns prepared by a computer are provided to an electron beam drawing equipment and physical interference fringes are formed by scanning an electron beam across a medium, has been put to practical use.
With the advance of computer graphics technologies, the handling of various images on computers is becoming a general practice in the printing industry. It will thus be convenient to be able to prepare original images, to be recorded as holograms, in the form of image data obtained using a computer. To answer such demands, arts for preparing computer holograms are becoming important and are anticipated to take the place of optical hologram preparing methods in the future. Various arts related to such computer holograms are disclosed in Japanese Patent Publications No. 11-024539A, No. 2001-109362A and No. 2003-186376A (hereinafter Patent Documents 1, 2 and 3).
Also, although “hologram” normally refers to an optical interference fringe pattern that enables reproduction of a three-dimensional pattern, recently, a medium called a “pseudo hologram,” in which a diffraction grating pattern is formed in place of an optical interference fringe pattern, has come to be used popularly. For example, Japanese Patent Publications No. 06-337622A, No. 07-146635A and No. 07-146637A (hereinafter Patent Documents 4, 5 and 6) disclose methods for preparing a “pseudo hologram,” with which a predetermined motif is expressed by arraying diffraction grating patterns of various types as pixels, by using a computer, and Japanese Patent Publication No. 2001-083866A (hereinafter Patent Document 7) discloses a method for recording such a “pseudo hologram” and a “normal hologram” on the same medium. Also, Japanese Patent Publications No. 2002-328639A and No. 2002-333854A (hereinafter Patent Documents 8 and 9) disclose examples of pseudo holograms that use scattering structure patterns instead of diffraction grating patterns.
As mentioned above, although a normal “hologram” refers to an arrangement with which optical interference fringes of an object light and a reference light are recorded on a medium, recently, media, with which various motifs are expressed by diffraction grating patterns or scattering structure patterns, are also coming to be referred to generally as “holograms.” Thus, in the present application, the term “hologram” shall be used as a broad concept that includes not only normal holograms, formed of optical interference fringe patterns, but also includes pseudo holograms formed of diffraction grating patterns (diffraction grating recording media) and pseudo holograms formed of scattering structure patterns (scattering structure recording media).
In a hologram for a cash voucher or credit card, various motifs, such as a company logo mark, a character string indicating a company name, etc., are recorded according to application. Methods for superposingly recording a plurality of motifs on the same hologram recording medium have thus been proposed. Because both normal holograms, in which optical interference fringe patterns are recorded, and pseudo holograms, in which diffraction grating patterns are recorded, have a function of making use of the diffraction phenomenon of light to generate reproduction light, directed in specific directions, two motifs can be recorded superposingly in a manner such that a first motif is observed upon observation from a first direction and a second motif is observed upon observation from a second direction. For example, the abovementioned Patent Documents 2 and 3 disclose methods for superposingly recording information of a plurality of original images onto the same recording medium, and the abovementioned Patent Document 4 discloses a method for superposingly recording diffraction grating patterns for indicating two different alphabetical characters.
When two motifs can thus be recorded superposingly, the two motifs can be displayed switchingly according to the observation direction so that, for example, a motif, constituted of a character string indicating a company name, is observed upon observation from a first direction, and a motif, constituted of a company logo mark, is observed upon observation from a second direction. However, depending on the application, such a method of switching according to observation direction may not necessarily be appropriate. For example, there are cases where it is preferable for both the character string indicating the company name and the company logo mark to be displayed next to each other at the same time.
Such cases are conventionally accommodated by simply positioning the two motifs adjacently. For example, the abovementioned Patent Document 7 discloses an art of recording a first motif as an optical interference fringe pattern onto a central region of a medium and positioning a second motif as a diffraction grating pattern at a peripheral region of the medium. However, when a plurality of motifs are simply positioned adjacently in this manner, the motifs do not blend well with each other and the resulting hologram lacks design quality.